All network activity failed in this computer. I tried all software fixes and concluded that the built-in CAT-5 network port on the mainboard was dead. So I added another CAT-5 card to an empty PCI slot. It still did not work. So I tried a different CAT-5 card and it failed again. I tried a total of 4 CAT-5 cards and the problem remained. Three of these cards had come from old computers, and one was new out of the box. I was about to give up and then said to myself, "Is it remotely possible that ALL FOUR cards were bad?" So I got another CAT-5 card out of an old computer, plugged it in, and POOF! It worked just fine.

Damn! Four bad cards out of five, and one of these was brand-new.

David F Mayer